Engine fuel control



Nov. l0, 1964 R. H. CORNELL 3,156,291

ENGINE FUEL CONTROL Filed Dec. 29, 1961 United States Patent O 3,156,29l ENGHJE FUEL CONTROL Richard H. Cornell, Marblehead, Mass., assigner to General Electric Company, a corporation of New York Filed Dec. 29, 1961, Ser. No. 163,330 6 Claims. (Cl. 153-36.4)

My invention relates to fuel control systems for gas turbine engines and in particular to an improved system for providing a preselected minimum fuel flow schedule.

In certain types of engine fuel control systems, particularly in the case of afterburner fuel controls, it is necessary to provide a minimum fuel ow schedule which speciiies a minimum fuel flow rate for the system under all conditions of operation. In the case of afterburner fuel systems, this requirement is typically generated by the type of fuel pumping and control system employed, which usually involves a centrifugal or similar type pump, the output flow rate of which is controlled by throttling.

In this type of system, when a reduction in fuel flow rate is desired, the amount of throttling on the pump output is increased, thereby causing a greater proportion of the pumping energy to go into heat and at the same time reducing the flow rate of the fuel which absorbs this heat. As the lower fuel flow rates are scheduled by increased throttling, the pump operating temperature will increase and at very low flows the pump temperature will begin to rise very rapidly causing the pump to approach a failure condition. In order to avoid the overheating problem, it is necessary to impose a minimum fuel flow requirement on the system which is capable of absorbing the throttling losses without an excessive rise in temperature.

Now in systems of this kind, it is common to schedule fuel flow by adjusting the flow area of a metering valve across which a constant pressure is maintained. The flow through the metering valve is thus made substantially directly proportional to the valve ow area. Constant pressure across the metering valve may be maintained in several ways such as, for example, by adjusting the liow area of a pressure regulating valve in series iiow relationship with the metering valve.

In one application of such a system to an afterburner fuel control with which l am familiar, the area of the metering valve is controlled by two inputs to the valve, one being a rotational position and the other being a longitudinal position of the valve stem. Two area controlling ports in the valve are overlapped in such a fashion that the flow area is established as the product of the two inputs to the valve or, in other words, such that the iiow area is proportional to the product of the rotational movement multiplied by the longitudinal movement of the valve. In this particular application, one of the signals is made proportional to the scheduled fuel ow rate divided by the compressor discharge pressure, a parameter commonly referred to as Wf/l3, and the other signal is made proportional to P3, such that the product of these two signals, which schedules the fuel flow area, is thus made directly proportional to the called for fuel ow rate.

The problem comes in trying to provide a minimum fuel iiow limit in this kind of environment. The metering valve has two inputs, one being the Wf/P3 input, and the other being the P3 input, each of which is capable of varying the iiow area of the metering valve. Therefore, an approach which would call for limiting the flow area of the metering valve to some preselected minimum level would involve a two dimensional limit problem. Assuming a system with maximum and minimum Wf/P3 stops, a minimum P3 stop would, in fact, provide a minimum fuel flow limit on the system.

3,1%,291 Patented Nov. l0, 1954 ICC The minimum P3 stop would have to be set, however, such that at minimum Wf/P3, the resulting fuel flow would not be less than the specified minimum level. This means, however, that the system is incapable of responding to actual values of P3less than that represented by the stop limit, even though at higher Wf/W3 settings fuel iiow rates greater than the minimum would otherwise have been scheduled at the lower actual P3 Values. To give an example, consider a system having a maximum WI-/P3 setting of ten units and a minimum Wf/ P3 setting of two units. Assume that the minimum fuel ow limit is to be two unitsof Wf. The minimum P3 stop must be selected at one unit of P3 to provide a fuel flow of at least two units at the minimum Wf/P3 setting of two units. The system is thus made insensitive to P3 values less than one unit even though P3 values down to 0.2 units would otherwise be permitted at the maximum Wf/P3 setting of ten units while still providing at least two units of fuel flow. Because the P3 stop thus has to be selected to provide at least the minimum fuel ow at the minimum Wf/ P3 setting, a portion of the operating range of the system which would otherwise be available at the higher Wf/ P3 settings is lost.

Rather than attempting to place a limit on the minimum iiow area of the metering valve itself, it has also been suggested that the metering valve be provided with a fixed area bypass, the area of which is selected to provide the minimum required fuel flow. The fixed area metering valve bypass, however, adds an increment of area to all metering valve settings and gives the valve a non-zero intercept in the relationship between fuel flow and P3 for any given Wr/P3 setting. What this means is that because of the fixed area in parallel with the metering valve, the total area which establishes the flow rate is no longer exactly directly proportional to the product of the two inputs to the metering valve because only the area of the metering valve itself is determined by this product and not the area of the bypass. This error becomes more pronounced at the smaller metering valve areas because in this region the bypass area represents a greater proportion of the total area.

In view of the foregoing, it is accordingly an object of my invention to provide a minimum fuel ow schedule system for a gas turbine engine control of the foregoing general type in which a true minimum fuel flow schedule is established without affecting the ability of the control to perform over the remainder of its operating range.

I accomplish this and other objects and advantages of my invention in one embodiment thereof by providing first of all a P3 stop on the metering valve representing a P3 level at or below the desired minimum ow level at the maximum Wf/ P3 setting. It will be appreciated, of course, that the provision of such a P3 stop would normally allow flows lower than the desired minimum at Wf/P3 settings less than the maximum setting. In combination with this I provide in one embodiment a fixed area bypass around the pressure regulating valve which is in series with the metering valve. The operating flow area ranges of the pressure regulating valve and the bypass around the pressure regulating valve are selected such that under minimum iiow conditions the pressure regulator bypass area becomes the ow determining area of the system. This is done by making the operating pressure drop across the pressure regulator valve very much larger than the drop across the metering valve, or in other words, making the operating area range of the pressure regulating valve much smaller than the operating area range of the metering valve.

This means that once the minimum flow condition is reached, as determined by the area of the pressure regulator bypass, further reductions in the area of the metering valve, up to a point, have almost negligible effect in reducing the flow rate. This allows the minimum P3 stop to be selected at a low enough level to permit operation over the full range of conditions while still retaining the minimum flow schedule.

My invention will be better understood and various other objects and advantages thereof will become apparent from the following description taken in connection with the accompanying drawings in which:

FIG. l is a schematic of a portion of an afterburner fuel control embodying my invention;

FIG. 2 is a cross-sectional view of the porting relationships of the metering valve shown in FIG. l, illustrating the shapes and overlapping relationships of the flow control ports;

FIG. 3 is a graphical presentation of fuel flow versus compressor discharge pressure characteristics showing the minimum fuel ow schedules of several types of controls including the embodiment of FIG. l; and

FIG. 4 is a fragmentary view of a portion of the system of FIG. 1 modified to incorporate an alternative embodiment of my invention.

Referring now to FIG. l, I show a schematic of a portion of an afterburner fuel control for a jet engine. In operation fuel is supplied to the system from an engine driven centrifugal or similar type pump, not shown, through a supply conduit 10. The fuel flows into a metering valve 11, a pressure regulating valve l2 and then out to the afterburner system through a discharge conduit 13. The flow of fuel from the pump is controlled by the throttling action of the metering valve and the pressure regulating valve in a manner later to be explained.

The metering valve 1i comprises a piston portion 14 having secured thereto a stem through which control of the valve is exercised. The piston portion la of the valve is formed of a cylindrical member l5 and a web 17 which is secured to the stem l5. The cylindrical member 16 is mounted for rotational and longitudinal movement in a cylinder It which is formed in lthe valve casing 19.

Formed in the upper portion of the cylindrical member i6 is a slotted port 20, which in operation cooperates with and overlaps a portion of a port 2l formed in the valve casing i9 and extending through the wall of the cylinder 18. FIG. 2 shows a View of the shapes of the ports 2t) and 2l looking into the port 21 in the right hand direction as shown in PEG. l and for a particular posiiton of the metering valve piston. In operation, fuel flows from the inlet conduit it! into an upper chamber 22 in the metering valve and from there through the area formed by the overlap of the ports 2@ and 2l into the port 2l.

The port 21 discharges into the pressure regulating valve 12 which comprises a piston Z3 slidably mounted in a cylinder 24 and spring loaded in the left hand direction by means of a spring ZS. The spring is mounted on a shaft 25 and engages at one end a washer 2 which is secured to the shaft 26 and engages at the opposite end a washer 28 which is slidable on the shaft and which engages a snap ring 29 mounted in the piston 23. The shaft 26 is secured against longitudinal movement in the casing i9 by an adjustment screw 3l, which is formed as a part of the shaft 26 and which ermits adjustment of the spring force. The shaft 2d is sealed in the casing B by means of an 0 ring 31B.

Formed in the valve casing 19 and extending into the cylinder 24 of the pressure regulating valve is a discharge port 13a which communicates with the discharge conduit t3 of the control system. The exposed flow arca of the port i361 is determined by the amount of overlap of the piston 23 with the port 30.. it will thus be seen that the fuel flow from the inlet conduit i@ `to the discharge conduit 13 must pass through two area controlling passages, the first being formed by the overi lap of the ports 2.@ and 2l in the metering valve 1l and the second by the overlap of the piston Z3 with the port Ba in the pressure regulating valve l2.

The function of the pressure regulating valve 12 in controlling the exposed flow area of the port 13a is to maintain a substantially constant pressure drop across the metering valve 1.1., that is from the inlet chamber 22 to the discharge port 2l.

The pressure on the upstream side of the metering valve, that is the pressure in the chamber 22, is applied to the left hand side of the pressure regulating valve piston 23 through passages 32 and 53, the passage 32 having a filter therein to prevent dirt particles and other impurities from affecting the operation of the piston 23. The pressure on the downstream side of the metering valve, that is the pressure in the discharge port 2l, is applied to the right hand side of the p'essure regulating valve piston 23 by reason of the connection of the port 2l to the cylinder 24.

Because in operation the pressure on the downstream side of the metering valve 1l will always be less than the pressure on the upstream side, the resulting pressure difference across the pressure regulating valve piston 23 will be in a direction to force the valve to the right Or, in other words, `in a direction opposing the force exerted on the piston by the spring 25. The piston 23 will thus seek an equilibrium position at which the pressure difference force balances the spring force.

Now it will be observed that for any given metering valve flow area, represented by a particular overlap of the ports Ztl and 2l, movement of the piston 23 to change the exposed flow area of the port 13a will produce an adjustment in the flow and consequently a change in the pressure drop across the metering valve. In other words,

ecause the metering valve and the pressure regulating valve are series connected throttling orifices which control the fuel flow rate of the fuel pump, a change in the flow area of either device produces a change in the pressure drop across the other. Thus if, for a given set of conditions, the ow area of the pressure regulating valve is increased by moving the piston .Z3 to the left, the flow rate will be caused to increase, thereby increasing the pressure drop across the Aetcring valve. Similarly, a decrease in the flow arca of the pressure regulating valve causes a decrease in the pressure drop across the metering valve.

rhe gradient of the spring 2S is relatively flat in the range of operation, and for purposes of explanation will be assumed to exert a constant force on the piston 23 in the left hand direction. The primary change in the force level on the piston Z3 then cornes from the adjustment of the pressure drop across the metering valve l. The greater the pressure drop, the greater the force on the piston and vice versa. The piston Z3 will thus move to a force equilibrium position at which the force exerted on the piston by the pressure difference across the metering valve exactly balances the force exerted by the spring Assuming as l have mentioned above that the spring gradient is relatively i'lat, a constant pressure drop is thus maintained across the metering valve 1i from the inlet chamber ZZ to the discharge port 2l, the magnitude of which is determined by the force exerted by the spring which is in turn adjustable by means of the screw 3l.

The fuel flow rate through the metering valve is thus rade substantially directly proportional to the flow area of the metering valve as established by the overlap of the ports 2Q and Zl. The amount of overlap of the ports 2t) and 2l is controllable in two dimensions, one being the rotational position of the cylindrical piston portion 16 and the other being its longitudinal position, which are established respectively by the rotational and longitudinal positions of the valve stem i5.

In the system illustrated, the Wf/P3 parameter is introduced as the rotational position of the metering valve. It will be recalled that this parameter is computed desired fuel weight flow Wf divided by the compressor discharge pressure P3. This signal is introduced by means of an arm 35 which engages a post 36 on a collar 37 which is attached to the valve stem 15. The rotational position of the metering valve stem is thus determined by the longitional to the Wf/P3 parameter.

he longitudinal position, on the other hand, of the valve stem 1.5 is determined by the magnitude of the compressor discharge pressure signal P3. The P3 signal is converted into mechanical form by two opposing bel lows 3S and 39, the upper one 38 being connected to compressor discharge pressure and the lower one, 39, being evacuated. The bellows and 39 are positioned in a chamber which is exposed to ambient pressure through a conduit 41. The bellows 38 and 39 are interconnected by a rod 42 which has secured to it a laterally extending pin 43. VBecause the eiects of ambient pressure force on the bellows 3S are balanced out by the equal and opposite force exerted on the bellows 59, the netl force generated by the two bellows is thus made proportional to the absolute level of compressor discharge pressure P3.

This force is imposed by the pin 43 on an output link 44 which is pivotally mounted at 4.5. The link 44 forms the flapper portion of an orice-ilapper arrangement which includes oriices 46 and 47 connected in series ilow relationship by a conduit itl as shown and to the fuel supply pressure in the metering valve in chamber 22. The pressure on the upper side of the metering valve piston 14; is thus determined by the supply pressure and the pressure on the lower side is determined by the pressure drop across the orifice 46, which is in turn controlled by the proximity of the flapper 44 to the orifice 47.

The operation of dapper type valves is well known in the art and the arrangement comprising the flapper ed and the series connected oriices 4d and 47 will therefore not be further explained except to state that the position of the flapper 44 controls the pressure drop across the oriiice 46 and hence the pressure diierence across the metering valve piston 14. Movement of the iiapper valve 44 further away from'the orifice 47' increases the pressure drop across the orice 46 and hence increases thedownward force exerted on the piston 14 while movement of the ilapper 44 closer to the orilice 47 decreases the downward force on the piston 14.

The apper link 44 is connected at its opposite end to a feedback spring t9 which in turn engages one end of a feedback link 55, which is pivotally mounted at 51 and which at its opposite end engages the stem l5 of the metering valve. Also secured to the dapper link de is a reference spring 52 which exerts an opposite moment on the link i4 and which, together with the feedback spring 49, establishes a force gradient against which the bellows force proportional to P3 is applied.

The system operates in the following manner to position the metering valve piston 14 in a longitudinal direction as a function of P3.

Assume an equilibrium condition with a fixed W/P3 input establishing a ixed rotational position of the valve stem l5. Assume further an increase in P3 which increases the downward 'force exerted on the liapper le by the pin d3 and causes the ilapperto move away from the oriice 47.

Movement' of the flapper la away from the orifice $7 causes an increase in the pressure drop across the orifice 46 and hence an increase in the downward force on the i piston 14. As the metering valve piston moves downward, the link Sti is caused to pivot to increase the force exerted by the spring i9 on the flapper ed, in a direction to balance the force caused by the P3 increase until it moves the :dapper t4 back to its starting position. At some point the system will thus reach a new equilibrium position with the piston 14 -moved downward by an amount proportional to the increase in the P3 signal.

Now, as I have pointed out above, the metering valve flow area, which is determined by the overlap ot the ports d 2li and 2l, is a function of both the angular and longitudinal positions of the metering valve piston ld. In other words, for any given P3, the valve can be rotated in response to the Wf/P3 input to increase or decrease the flow area, and for any given Wf/P3, the valve can be moved longitudinally in response to the P3 input to increase or decrease the area. The total ilow area of the metering valve lll. is thus proportional to the product of Wf/P3 and P3 or, in other words, proportional to the desired fuel flow rate W3.

The resulting characteristic of fuel Weight ow Wf ver sus compressor discharge pressure P3 Afor constant Wf/ P3 is shown in FIG. 3 by the solid lines 5d and 55, the line 54 representing the maximum vili/P3 signal and the line 55 representing the minimum vili/P3 signal. ln other words, the line 5d represents the Wf versus P3 characteristic for a ixed rotational position of the metering valve corresponding to the maximum W3/P3 signal, the variation in Wf as a function ot P3 thenbeing determined by the longitudinal movement of the metering valve. The line 55 shows the same kind oi characteristic except for the minimum Wf/P3 signal. The total characteristic of the system is represented by the whole family of slopes falling between the lines 54 and 55.

l will now use the characteristics shown in FlG. 3 to discuss several approaches to the minimum fuel flow schedule problem in order to demonstrate the principles involved. Assume first oi all that the minimum fuel flow rate permissible is represented by the dotted line Se. Assume further that the approach selected is to limit the minimum value of metering valve tlow area by using a minimum P3 stop. The P3 stop may be accomplished by means of a housing stop 57 as l have illustrated in FlG. l which engages the metering valve piston le, although this stop is set at a diierent value than is the case with the example now under consideration.

At any rate, with the approach under consideration, the minimum P3 limit must be selected such that at the minimum W/ P3 setting, represented by the line 55, the scheduled fuel flow rate will not fall below that represented by the line 55. On this basis, the lowest value of the P3 limit that can be selected is represented by the dotted line 55. This'rneans that all actual values of P3 less than that represented by the line 5S will still result in the introduction of the minimum P3 signal represented bly the line 55, even though the actual P3 level is less than t at.

It will be observed, however, that the minimum P3 limit represented by the line 5S is higher than necessary for the hir/her W3/P3 signals. For example, at the maximum Wf/P3Ksignal represented by the line 54, the minimum fuel tlow requirements can be met with a minimum P3 as low as that represented by the line 59. With the P3. `stop set this low, however, flows less than the required minimumV would occur at vili/P3 settings less than that represented by the line 54. With the approach under discussion, then, the stop'must be selected to correspond to the line 52S with the result that the system is insensitive to P3 over the range between the lines 5S and 59 and above the line 56, a range where satisfactory operation could otherwise be obtained.

'Consider now an alternative approach in which a fixed area bypass is used around the metering valve, the bypass area being selected to supply the minimum flow rate represented by the line 56 with zero llow area .of the metering valve itself. This approach produces the character1st1cs represented by the lines 54st and 55a in which the zero intercept is moved up to the level ot the line Se.

The error introduced by this approach, particularly'at the lower P3 range at which the metering valve area begins to approach that of the bypass, is obvious from the illuse tration of FIG. 3.

With this background, l will now explain the approach which I follow in the embodiment of my invention shown vin FIG. l. ln this embodiment, l provide a P3 stop 57 amasar rset at a minim-.un P3 signal lei/ei corresponding to the iine 59 in PIG. As I have mentioned above, this approach, taken by itself, will allow flows less than the required minimum at Wf/P3 settings less than the maximum represented by the line 54. In combination with this, then, I provide a fixed area bypass valve 6i? around the pressure regulating valve 12. The bypass valve et) is connected to the metering valve discharge port 2l by means of an internal passage 6l and a conduit 62 and to the discharge conduit 13 of the pressure regulating valve I?. by means of a conduit e3.

Now in the typical case, the pressure drop across the pressure regulating valve is substantially greater than the drop across the metering valve. To give an example, in one application with which I am familiar, the pressure drop held across the metering valve is about 60 p.s.i. whereas the operating range of pressure drops across the pressure regulating valve is in the order of 650-700 p.s.i. In other words, in the operating range, the flow area of the pressure regulating valve l2 is selected as substantially smaller than ythe flow area range of the metering valve. I maintain that relationship here.

With the foregoing in mind, consider now the operation of the embodiment of FIG. l with the P3 stop 57 set to provide a P3 limit as shown at 59 in FfG. 3 and with the fixed area bypass 60 across the pressure regulating valve 12. For purposes of explanation, assume a minimum Wf/P setting as represented by the line 55. Assume further that P3 is being reduced with Wf approaching the minimum flow line 56. would normally continue to decrease below the level of 56 along the line 55.

In the vicinity of the intersection of the line 55 with the minimum fuel flow line Se, however, the pressure regulating valve is made to begin to lose its ability to control the pressure drop across the metering valve because the operating area of the pressure regulating valve begins to approach that of the bypass. At the intersection of line 5S with the minimum fuel iiow line 56, the pressure regulator is fully closed and bypass 60 establishes the minimum flow rate 56.

Because the area of the bypass et) is substantially less than the flow area of the metering valve in this region of operation, the area of bypass e@ determines the ilow rate and further decreases in the area of the metering valve have almost negligible effect on the flow rate in this region. It will be apparent, of course, that as the metering valve area continues to decrease, eventually that area will become smaller than the area of the bypass 6th and the metering valve area will then become dominant in controlling the flow rate. The minimum P3 stop is set, however, at a level such that the metering valve area will not go down into this range. The stop 57 is set at the P3 level represented by the line 59 which allows the full use of the operating map above the minimum fuel flew line 56. Actually, the stop can be set at a lower level of P3 without getting into the sensitive range, or it may be set at a higher level in the event it is not considered necessary to operate the control over some portion of the available range.

The point is, however, that my invention permits a P3 stop setting at a lower level than that represented by the line 58, thus permitting operation over a wider range of the available map.

It will be appreciated by those skilled in the art that the minimum Wf line S6 which runs from the P3 stop line 59 to the intersections with the constant Wf/Pa lines actually has a very small upward slope blending into each of the constant Wf/Pg lines. In other words, varia.

tions in metering valve flow area produced by changes in P3 in this region actually produce a very small effect on the fuel ilow rate Wf. Because of the large difference in area, however, between the metering valve and the bypass 60, this effect is virtually negligible. Once the P3 stop is Except for the bypass 60, Wr i' engaged at the line 59, the minimum fuel flow line is then essentially fiat back to zero actual P3 because no further area variations take place.

In FIG. 4, I show an alternative embodiment of my invention in which a pressure regulating valve minimum area stop 64 is used instead of a bypass around the pressure regulating valve. This provides essentially the same characteristic as that produced by the embodiment of FIG. l in that a fixed minimum pressure regulating valve area is provided as the lower P3 values are approached. The stop 64- may be threaded into the casing 19 as shown to permit adjustment to any desired area setting. At some preselected iiow area of the pressure regulating valve, corresponding to that required for accommodating the minimum fuel flow rate, the stop 64 engages the piston 23 to fix the minimum area of the valve. As in the case of the embodiment of FIG. l, this area is much less than the area of the metering valve in this range of operation such that over this range and down Ito the minimum P3 stop, the minimum fuel flow rate is established by the area xed by the stop 64, with changes in the area of the metering valve l1 producing only a negligible effect.

It will be appreciated, of course, that various other bypass and minimum area stop arrangements may be used on or with the pressure regulating valve to achieve the results which I have described. The significant point is that at the lower P3 values approaching the minimum fuel flow limit, the pressure regulating system is caused to saturate at a fixed flow area connected in series with the metering valve, this series connected flow area then substantially determining the minimum fuel iiow for further decreases in P3 down to the P3 stop limit, thereby permitting a P3 stop at a lower level than would otherwise be permissible.

Various other applications of my invention will occur to those skilled in the art and it should thus be understood that various modifications, changes and substitutions may be made in the embodiments set forth without departing from the Itrue scope and spirit of my invention as I have defined it in the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A gas turbine engine fuel control system comprising f a fuel pump, a metering valve connected in series flow relationship with said fuel pump for controlling the output ow rate thereof, a pressure regulating valve connected in series flow relationship with said metering valve for maintaining a substantially constant pressure drop across said metering valve; and means for establishing a minimum fuel flow schedule comprising:

(a) minimum flow area limiting means establishing a preselected minimum flow area in series with said metering valve, and

(b) means establishing a variable preselected minimum flow area of said metering valve,

(c) 'the minimum flow area established by said minimum flow area limiting means being less than the minimum ilow area of said metering valve.

2. A gas turbine engine fuel control system comprising:

(a) fuel pumping means,

(b) a metering valve connected in series flow relationship with said fuel pumping means for controlling the output iiow rate thereof and including means for limiting the flow area of said metering valve to a variable preselected minimum area,

(c) a pressure regulating valve connected in series iiow relationship with said metering valve for maintaining a substantially constant pressure drop across said metering valve, and

(d) a fluid tiow bypass connected in parallel with said pressure regulating valve for establishing a minimum fuel flow rate for the system.

3. A gas turbine engine fuel control system comprising:

(a) fuel pumping means,

(b) a metering valve connected in series How relationship with said fuel pumping means for controlling the output ow rate thereof and including means for limiting the flow area of said metering valve to a variable preselected minimum area,

(c) a pressure regulating valve connected in series flow relationship with said metering valve for maintaining a substantially constant pressure drop across said metering valve, and

(d) means for limiting the ow area of said pressure regulating valve to a preselected minimum level, thereby establishing a minimum fuel flow rate for the system.

4. A fuel control system for a gas turbine engine comprising a fuel pump, a metering valve connected in series How relationship with the fuel pump for controlling the output ow rate thereof, a pressure regulating valve connected in series flow relationship with said metering valve for maintaining a substantially constant pressure drop across said metering valve; and means establishing a minimum fuel flow schedule comprising:

(a) a fluid ow bypass connected in parallel with said pressure regulating valve, and

(b) stop means associated with said metering valve and establishing a variable minimum flow area thereof,

(c) the flow area of said uid flow bypass being less than the minimum How area of said metering valve as established by said stop means.

5. A fuel control system for a gas turbine engine comprising a fuel pump, a metering valve connected in series with the fuel pump for controlling the output flow rate thereof, a pressure regulating valve connected in series ow relationship with said metering valve for maintaining a substantially constant pressure drop across 10 said metering valve; and means establishing a minimum fuel ow schedule comprising:

(a) irst stop means associated with said metering valve and establishing a variable minimum ow area thereof, and

(b) second stop means associated with said pressure regulating valve and establishing a minimum flow area thereof.

(c) the minimum flow area of said pressure regulating valve as established by said second stop means being less than the minimum flow area of said metering valve as established by said first stop means.

6. A gas turbine engine fuel control system comprising:

(ar) a fuel pump,

(b) a metering valve connected to control the flow rate of fuel from said fuel pump to a combustion system,

(c) variable ow area pressure regulating means connected in series flow relation with said metering valve for maintaining the pressure drop across said metering valve substantially constant, and

(d) means for saturating the pressure regulating characteristic of said pressure regulating means at a preselected minimum fuel flow rate corresponding to a variable preselected minimum flow area of said metering valve,

(e) the flow area of said pressure regulating means in its saturated condition being less than said preselect-- ed minimum metering valve iiow area.

References Cited in the tile of this patent UNITED STATES PATENTS 2,845,087 Thomas July 29, 1958 2,879,643 Stroh et al. Mar. 31, 1959 2,950,733 IPerkins Aug. 30, 1960 3,106,934 Rogers e't al Oct. 15, 1963 

6. A GAS TURBINE ENGINE FUEL CONTROL SYSTEM COMPRISING: (A) A FUEL PUMP, (B) A METERING VALVE CONNECTED TO CONTROL THE FLOW RATE OF FUEL FROM SAID FUEL PUMP TO A COMBUSTION SYSTEM, (C) VARIABLE FLOW AREA PRESSURE REGULATING MEANS CONNECTED IN SERIES FLOW RELATION WITH SAID METERING VALVE FOR MAINTAING THE PRESSURE DROP ACROSS SAID METERING VALVE SUBSTANTIALLY CONSTANT, AND (D) MEANS FOR SATURATING THE PRESSURE REGUALTION CHARACTERISTIC OF SAID PRESSURE REGULATING MEANS AT A PRESELECTED MINIMUM FUEL FLOW RATE CORRESPONDING TO A VARIABLE PRESELECTED MINIMUM FLOW AREA OF SAID METERING VALVE, (E) THE FLOW AREA OF SAID PRESSURE REGULATING MEANS IN ITS SATURATED CONDITION BEING LESS THAN SAID PRESELECTED MINIMUM METERING VALVE FLOW AREA. 